Some medical X-ray imagers have X-ray tubes with rotating anode disks and a further dynamic feature that allows changing by means of electric or magnetic fields the direction of the electron beam incident on the rotating anode. This dynamic focal spot movement helps increase image quality, that is less noise and higher resolution are achievable without increasing power or decreasing focal spot size. However it has been observed that such imagers require frequent maintenance when run at relatively high tube power. An imager with a rotatable anode is described in US 2011/0280376.